The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
During a combustion cycle of an internal combustion engine (ICE), air/fuel mixtures are provided in cylinders of the ICE. The air/fuel mixtures are compressed and combusted to provide output torque. After combustion, pistons of the ICE force exhaust gases in the cylinders out through exhaust valve openings and into an exhaust system. The exhaust gases may contain oxides of nitrogen (NOx) and carbon monoxide (CO) and hydrocarbons (HCs).
An exhaust gas treatment system of the ICE may include a catalytic converter and a selective catalytic reduction (SCR) catalyst to reduce exhaust emissions. As an example, a thee-way catalyst converter (TWC) may be used to reduce NOx, CO and HCs within an exhaust system. The TWC converts NOx to nitrogen and oxygen, CO to carbon dioxide and oxidizes unburnt HCs to produce carbon dioxide and water. A SCR catalyst may be located downstream from the TWC and may further reduce NOx in the exhaust system. A SCR catalyst converts NOx into nitrogen N2 and water H2O.
An average light-off temperature at which a catalytic converter typically begins to function is approximately 200-350° C. An average temperature above which a SCR catalyst is active is also approximately 200-350° C. As a result, a catalytic converter and a SCR catalyst do not function or provide minimal emission reduction when their temperatures are not maintained at or above respective light-off and/or minimum active operating temperatures.
In order to increase and maintain temperatures of the catalytic converter and the SCR catalyst at or above the light-off and/or minimum active operating temperatures, lean operation of an ICE is limited. An ICE may be operated with a lean air/fuel ratio to minimize fuel consumption and improve operating efficiency of the ICE. The leaner the ICE is operated, the less fuel consumed and the more efficient the ICE operates.
However, the leaner the ICE is operated, the lower the operating temperatures of the ICE, which decreases the amount of thermal energy transfer to exhaust system components. As thermal energy to the exhaust system decreases, the temperatures of the catalytic converter and SCR catalyst can decrease to below active operating temperatures. For this reason, lean operation is limited to maintain catalytic converter and SCR catalyst operating temperatures above active operating temperatures. Limiting lean operation limits the amount of reduction in fuel consumption.